Recently, demand for high-speed data transfer is increasing, and the OFDM suits with this high-speed data transfer and is employed in various high-speed communication systems. Hereinafter, the OFDM will be described. A basic principle of the OFDM is that a data stream having a high-rate is divided into a plurality of data streams having a slow-rate that are transmitted by a plurality of carriers simultaneously. Each of the plural carriers is called a sub-carrier. Since there is orthogonality between the plural carriers of the OFDM, a frequency component of the carrier can be detected at a receiving end even when the carriers are overlapped with each other. The data stream having a high-rate is transmitted to the receiving end by which the data stream is converted into a plurality of data streams having a low-rate by a serial-to-parallel converter, the sub-carriers are multiplied to the parallel converted plural data stream, and the multiplied converted data streams are combined with each other.
An orthogonal frequency division multiple access (OFDMA) is a multiple access method of allocating the sub-carriers to entire broadband in the OFDM according to transfer rate demanded by multiple users.
There is proposed a single carrier frequency division multiple access (SC-FDMA) technique of adding a spreading technique by a discrete Fourier transform (DFT) spreading matrix to the OFDM technique. The SC-FDMA technique has a low Peak-To-Average Power Ratio (PAPR).
The OFDM communication technique is a communication method used in various systems such as IEEE 802.11a/g, HiperLAN, IEEE 802.16, Digital Subscriber Line (DSL), Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and the like and is effective when a communication channel appears as a selective-fading phenomenon. The OFDM communication method uses several sub-carriers so that the selective-fading looks like a flat fading and has a merit that a technique of compensating the fading is simplified in overall system. In order to easily estimate the simplified channel, pilot sub-carrier information is used. Since a position and a value of the pilot sub-carrier are already known to a transmitting end and a receiving end, the receiving end can obtain the position and value of the pilot sub-carrier simply by carrying out division (or equivalent operation) in order to estimate the channel.
For the synchronization, a phase difference between different OFDM symbols is determined, and this is obtained by comparing a phase of a pilot signal with the different OFDM signals.
This pilot may be used in various purposes other than the above-mentioned two purposes, for example, in an encoded pilot or in a technique of reducing the PAPR.
FIG. 1A illustrates a conventional transmitting end to transmit an OFDM signal. As illustrated in the drawing, data bits are mapped into specific data symbols according to constellation mapping, and pilot bits are mapped into specific pilot symbols according to constellation mapping. The data symbol is mapped into sub-carriers after being converted into a parallel signal (a transfer symbol to be transmitted to the receiving end) through the serial-to-parallel conversion. The data symbol is transmitted to the receiving end after carrying out an inversion fast Fourier transform (IFFT). The data bits represent a bit stream indicating user data except for the pilot sub-carrier.
The operation is expressed by the formulas as follows.
In the conventional OFDM transfer technique, when N sub-carriers are used, Np pilot sub-carriers among the N sub-carriers are allocated and the rest is allocated to data or a guard band. Hereinafter, for the illustrative convenience, the data (the user data and the pilot data) except for the guard band will be described. When the number of the sub-carriers allocated to the data is Nd, a relationship N=Np+Nd is established. A vector of the transfer symbol in which the user data is combined with the pilot signal, =[S0, S1, Λ, SN-1]T is expressed by the following formula.=Pd+Pp   [Formula 1]
where, Pd and Pp are matrixes of re-arranging the user data and the pilot sub-carriers at positions of the already allocated sub-carriers, and the  and  are the symbol vectors transmitted by the user data and the pilot sub-carriers respectively and the lengths thereof are Nd and Np respectively. The transfer symbol vector  in the frequency region meets with the IFFT like in the following formula.=[x0,x1,Λ,xN-1]T=F−   [Formula 2]
where, F is a Fourier transform matrix. The vector  is modulated into a carrier frequency to be transmitted through an antenna again, and the receiving end receives a signal such as =+. Here,  is a response vector of a wired/wireless channel and  corresponds to a noise. The receiving end firstly carries out the Fourier transform in order to demodulate the vector  representing the receiving signal. Then, the following formula 3 is expressed.=F=H+F   [Formula 3a]
where, H represents a channel response within the frequency region and  represents original data. If the channel has been estimated, the transfer signal is demodulated by the following formula according to the estimated channel value.=(HHH)−1HH   [Formula 3b]
where,  is an estimated value of . A conventional method of transmitting the OFDM signals is identical to the above-mentioned transfer method or is carried out by processes corresponding to the same.
Hereinafter, the PAPR causing a problem in the OFDM communication system will be described.
In the OFDM communication system, there occurs a problem such that a linear span of a power amplifier of an transmitting end must be wider as the PAPR is high. In general, since a power amplifier having a large linear span is expensive, in order to reduce manufacturing costs of mobile terminals, a cheap power amplifier is used in wired/wireless communication so that an output range is narrow and due to this, the OFDM signal is distorted.
Various methods have been proposed in order to solve the problems about the PAPR, and are grouped into two parts. A first method is to transmit additional information for PAPR identification (for example, a selective mapping, a partial transmit sequence, and the like), and transmit the additional information for PAPR identification through an additional channel by forming the additional channel in a way of using some of the sub-carriers. A second method does not need the additional information for PAPR identification (for example, a tone reservation), and in this case, the user consumes more electric power and a receiving end undergoes more interference.
FIG. 1B is a view illustrating a conventional transmitting end to reduce the PAPR using the additional information for PAPR identification.
In the conventional transmitting end illustrated in FIG. 1B, the OFDM signal is transmitted according to the formulas 1 and 2.
A PAPR reducing technique has a target of minimizing a difference between an average value and a maximal value of power of  in Formula 2. The PAPR is defined by the following formula.
                              P          ⁢                                          ⁢          A          ⁢                                          ⁢          P          ⁢                                          ⁢          R                =                                            max                                                k                  =                  0                                ,                Λ                ,                                  N                  -                  1                                                      ⁢                                                                            x                  k                                                            2                                                          1              N                        ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                                                      x                    k                                                                    2                                                                        [                  Formula          ⁢                                          ⁢          4          ⁢          a                ]            
As shown in the above formula, when any one of the vector components has an abnormally large value, the PAPR is increased and the characteristic of the signal is deteriorated. To solve this problem, a method used in the frequency region can be expressed by the following formula.=MSMP   [Formula 4b]
where, Ms is a matrix of varying a phase component of the respective data components of , and Mp is a matrix of changing order of the data components. This new modified data vector  is transformed into a signal in a time domain through a transform such as the formula 2 and the PAPR of a signal transformed into the time domain.
Since there are several Ms and Mp, signals are made in a time domain and the smallest PAPR is selected from the signals and is transmitted. At that time, the additional information about the Ms and Mp must be transmitted and this information is called the additional information for PAPR identification.
The additional information for PAPR identification can be transmitted by generating additional channel in a code division multiple access (CDMA) communication system. The OFDM communication system adopts a method of being allocated with some of the sub-carriers to transmit the additional information for PAPR identification.
The conventional methods of transmitting the additional information for PAPR identification must use the additional channel for the transmission of the additional information for PAPR identification.
If some of the sub-carriers of the OFDM communication system are used as the additional channels, the communication system may be deteriorated.
Moreover, in the conventional methods of not transmitting the additional information for PAPR identification, the transmitting end's total power is increased and interferes with other receiving ends.